The soldier of the ant Camponotus (Colobopsis) fraxinicola, as a trophic caste.

144

The Soldier of the Ant,

Camponotus (Colohopsis) fraxinicola,

as a Trophic Caste

BY E. O. WILSON

Reprinted from Psyche, Vol. 81, No. 1, March 1974
pages 182-188

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THE SOLDIER OF THE ANT

CAMPONOTUS (COLOBOPSIS) FRAXINICOLA

AS A TROPHIC CASTE*

By Edward O. Wilson

Museum of Comparative Zoology Laboratories,

Harvard University, Cambridge, Massachusetts 02 138 U.SA.

Complete dimorphism, defined as the coexistence of a minor worker
of ordinary proportions with a larger major worker or "soldier,"
represents the pinnacle of the subcaste system within the ants. The
soldier not only weighs more but also possesses a disproportionately
larger head. By definition, intermediate forms are lacking. Complete
dimorphism has originated independently at least seven times, within
the following genera: the myrmicines Pheidole, Oligomyrmex,
Acanthomyrmex, Paracryptocerus; the dolichoderine Zatapinoma;
and the formicines Camponotus and Pseudolasius. The head shapes
of the major workers of these groups are clearly modified either for
fighting or for defense of the colony by blocking the nest entrances.
These functions have been confirmed by direct observations of col-
onies of Pheidole, Paracryptocerus, and Camponotus (Wilson, 1971).
The behavioral repertory of the major workers is otherwise very
limited in comparison with that of the minor workers, giving logic
to their alternate designation in the literature as soldiers.

The principal purpose of this article is to demonstrate that in at
least one species, Camponotus (Colobopsis) fraxinicola M. R. Smith,
the soldier caste also plays a key role in liquid food storage.

Materials and Methods

Colonies were collected at St. Mark's Lighthouse and Tall Tim-
bers, near Tallahassee, Florida. The taxonomic identification re-
quires a brief note. Three forms of the southeastern United States,
fraxinicolaj impressus, and pytartes, are very similar to each other
and may prove synonymous. The Tallahassee series fall closest to
fraxinicola in the seemingly best character states of the major worker:
a slightly more flattened mesonotum, more rounded posterior rim of
the truncated portion of the head, and in other, more subtle details
of head shape. But all of these characters vary widely within and
among series 'from widespread localities, so that eventually only one

*Mam'script received by the editor February 28, 1974.

182

1974] Wilson — Camponotus fraxinioola 183

species might be recognizable, which would then take the name
impressus.

The colonies were transferred to glass tubes 15 cm in length and
3-6 mm in inner diameter. The tubes were each plugged at one end
with dry wads of cotton wool and placed in open plastic containers,
the inner sides of which were lined with fluon to discourage climbing
by the ants. Since the latter method does not always work with these
highly arboreal ants, the containers were also supported by glass
bottles set in petri dishes filled with heavy mineral oil. Water was
made available in moistened cotton plugs at the bottom of test tubes
placed next to the tubes housing the colonies. The ants were fed
daily with honey and freshly killed insects. Entire colonies quickly
habituated to strong light. They could be observed in toto with a
swing-arm dissecting microscope without being disturbed in their
new, highly simplified (and fluon-lined) universe.

The Soldier as a Defensive Caste

Since the time of Forel (1874) it has been known that soldiers of
the subgenus Colobopsis of Camponotus use their oddly cylindrical
heads to block the nest entrances. Minor workers returning from
foraging trips identity themselves to the soldiers, presumably by
colony odor. The soldiers then pull back to let them enter. C. frax-
inicoldj like most other Colobopsisj nest in the cavities of dead twigs.
The nest entrances are neat, circular holes into which the heads of
the soldiers fit snugly. By plugging glass tubes containing colonies
with disks of cork, I was able to observe the excavation of the nest
entrances on repeated occasions. The task was performed exclusively
by the minor workers. This is somewhat surprising in view of the
fact that the holes are cut to fit the specifications of the soldiers and
not those of the minor workers. The soldiers also rested more con-
sistently near the entrances of the glass tubes, with their heads point-
ing outward in a high proportion o'f cases, even when they were not
actively engaged in blocking the entrances.

Undecane, stored in Dufour's gland of the abdomen, is a general
formicine alarm pheromone (Wilson and Regnier, 1971). When
small quantities of this substance were allowed to evaporate near the
nest entrance, all members of the fraxinicola colonies were thrown
into the typical excited running movements of the fraxinicola alarm
response. But some of the soldiers moved to the nest entrances, fill-
ing those holes still unplugged at the start of the alarm reaction.

1 84

Psyche

[March

Figure 1. Portion of a laboratory colony of Camponotus [Colobopsis]
fraxinicola housed in a glass tube. The large soldiers use their cylindrical
heads to block the circular nest entrances, which are cut by the minor
workers. This colony has been fed ad libitum, and consequently both soldiers
and minor workers are in replete condition. In the center, a soldier and
minor worker exchange food by regurgitation.

When twigs containing fraxinicola colonies are first broken open,
both minor workers and soldiers rush out. Many attack any accessible
alien object, such as the observer's hand or a bit of cloth offered to
them, biting it and spraying it with formic acid. The same response
was obtained in the laboratory by permitting fire ant workers {Sol-
enopsis invicta) to invade the nests. Individuals of both castes were
about equally aggressive and effective in repelling these invaders. On
the other hand, the total population of minor workers, by virtue of
its greater size, was more effective than that of the soldiers.

To sum up the results, the fraxinicola soldiers are indeed a defen-
sive caste, but their specialization makes them superior in only one
aspect of this role.

The Soldier as a Trophic Caste

The fraxinicola major worker is also anatomically distinguished
by its proportionately larger abdomen. All individuals dissected from
two laboratory colonies had large fat bodies and well developed
ovaries containing one to seven eggs, some of which were of very

1974] Wilson — Camponotus fraxinicola 185

large size. A few minor workers with distended abdomens taken
from within the nest had similar abdominal contents, but the ma-
jority from both within and outside the nest had proportionately
much smaller fat bodies and reduced ovaries. Thus the soldiers store
substantially more food in the form of fat than do the minors. The
nature of their eggs has not been ascertained. Should they prove to
be trophic eggs, this form of storage would be primarily the province
of the soldier caste.

The following data reveal that the soldiers also store dispropor-
tionately large quantities 6i liquid food in their crops. When colonies
are fed ad libitum with saturated sucrose solution or honey, most of
the workers in both subcastes become repletes. Repletism has been
defined for purposes of this analysis as distention of the abdomen to
the extent that the intersegmented membranes are exposed, permitting
the interior of the crop to be seen when light is transmitted from
below. It has been repeatedly observed in our laboratory colonies
that a higher proportion of the major workers — sometimes all of
them — become repletes when the colony is offered a superabundance
of liquid food. But when the colony is starved, the proportion of
repletes among the majors drops below that of the minors. Two
examples are presented in Figure 2.

The amount of liquid stored by each major is substantially greater
than that stored by each minor, on both an absolute and per-unit-
weight basis. This difference, which was first guessed by simple in-
spection, was proved by the following series of measurements. Twenty
minors and 11 majors were selected at random and weighed from a
colony anesthetized after two weeks of starvation. Then the colony
was fed to satiety over a 24-hour period and anesthetized again; 20
minors and 16 majors were next selected at random from among the
replete individuals and weighed. The mean weight of the starved
minors was 1.62 mg (range 1.29-2.42 mg) ; after feeding, their mean
weight was 2.44 mg (range 1.75-3.33 mg)) a gain of 0.82 mg or
50.6 percent. The mean weight of the starved majors was 3.44 mg
(range 2.38-4.65 mg) ; after feeding, their mean weight was 5.62
mg (3.14-7.34 mg), a gain of 2.18 mg or 63.4 percent.

The disparity in storage capacity can be seen even more clearly by
examining the colony as a whole. The colony labelled No. i in
Figure 2 can be taken as typical; it contained 139 minors, 26 majors,
and a single queen. Using the data on weight gain and percentage of
repletism during a single experimental run, the following storage
capacities were estimated: the entire minor population stored 88.52

i86

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Figure 2. The frequency of repletes among the soldiers and the minor
workers respectively in two colonies of Camponotus (Colobopsis) fraxini-
cola. The ants were fed to satiation with sucrose solution on three occa-
sions ten days apart and starved in the intervening periods. The soldier
caste consistently achieved a higher level of repletion when the colony was
fed lull siurrendered it to a greater extent when the colony was starved.

1974] Wilson — Camponotus fraxinicola 187

mg, and the entire major population stored 54.59 mg. Thus although
the majors made up a little less than 16 percent of the population and
contained 28.43 percent of the wet weight in the non-replete condi-
tion, they stored 38.15 percent of the liquid at repletion.

A distinctive history of liquid flow unfolds when a colony dis-
covers a single rich source of food and then endures a period of
starvation — the circumstance simulated by the laboratory experi-
ments. When the food is first discovered, the flow during the first
one or two hours is from the foraging minor workers to other minors
and majors encountered back at the nest. After saturation is attained,
and go percent or more of the adults are replete, regurgitation con-
tinues at a high rate. The two castes participate at about the same
per-worker rate, with no apparent difference between the majors and
minors in the frequency of donation as opposed to that of reception.
Within 2-3 days after the food is cut off, majors have begun to feed
minors larger quantities than they receive. Data on exchanges show
that the major-to-major donations are fewer than would be expected
by chance alone. This is due at least in part to the fact that the
majors are relatively sluggish in their movements. The minors are
much the more active caste, passing from one nestmate to another to
collect and pass along the dwindling supply of liquid food.

Acknowledgement

This article is part of a, continuing study on caste systems sup-
ported by Grant Number GB-40247 from the National Science
Foundation.

Summary

(i) The major worker, or "soldier," of Camponotus (Colobopsis)
fraxinicola, helps to defend the nest by blocking the entrance holes
with its head, a behavior pattern reported by previous authors in
other members of the genus. However, when the nest is breached and
combat ensues, the soldier is no more aggressive or effective than the
minor worker.

(2) The nest entrances are constructed entirely by the minor
workers, which cut them to fit the cylindrical heads of the majors.

(3) Major workers respond to undecane, a general formicine
alarm substance, by moving to block those nest entrances still open
at the beginning of the episode.

1 88 Psyche [March

(4) Major workers also serve as a storage caste. Their abdomens
are partially filled with exceptionally large fat bodies. They also
store a disproportionate share of sugary liquids collected by the col-
ony. When this food is superabundant, the majors reach a higher
level of repletion. When the colony is subsequently starved, the
majors regurgitate the liquid back to the rest of the colony faster
than they receive it.

Literature Cited

FOREL, A.

1874. Les fourmis de la Suisse. Societe Helvetique des Sciences Natu-
relles, Zurich, iv + 452 pp.
Wilson, E. O.

1971. The insect societies. Belknap Press of Harvard University Press.
I + 548 pp.
Wilson, E. O. and F. E. Regnier.

1971. The evolution of the alarm-defense system in the formicine ants.
American Naturalist, 105: 279-289.